auxotrophs

An auxotroph is a mutant organism that cannot synthesize a specific compound required for growth and therefore must obtain that compound from its environment. By contrast, a prototroph can produce all essential nutrients from basic media. Auxotrophy typically results from mutations in genes encoding enzymes of biosynthetic pathways, such as those for amino acids, nucleotides, vitamins, or cofactors.

Common examples include amino acid auxotrophs (for example leucine leu-, tryptophan trp-, or histidine his- in bacterial or fungal strains), nucleotide auxotrophs (such as ade- or ura- in yeast), and vitamin auxotrophs (biotin or thiamine requirements in certain mutants). In laboratory settings, auxotrophs are usually grown on minimal media that provide only essential salts and a carbon source; growth is allowed only if the missing nutrient is supplied or if the organism carries a functional gene complement.

Auxotrophs are valuable in genetic research and biotechnology. They enable selection and screening through complementation or marker rescue: introducing a plasmid or construct carrying the missing gene can restore prototrophy, allowing transformants to be selected by their ability to grow without the supplemented nutrient. They are widely used as selectable markers in organisms such as bacteria and the yeast Saccharomyces cerevisiae. Auxotrophic strains underpin genetic mapping, functional genomics, and the construction of deletion libraries, as well as metabolic engineering efforts to study pathway functions and regulation.